In recent years, a spark plug used in an internal combustion engine, such as an automobile engine, has a noble metal chip welded to an ignition surface of a front end portion of a center electrode or to an ignition surface of a ground electrode which faces the center electrode, for the purpose of enhancing resistance to spark-induced erosion. The noble metal chip is formed from a noble metal having excellent resistance to spark-induced erosion, such as platinum (Pt), palladium (Pd), or iridium (Ir), or from an alloy which predominantly contains such a noble metal. Meanwhile, a metal having good thermal conductivity, such as a Ni alloy, is used as an electrode base metal of the center electrode or the ground electrode to which the noble metal chip is joined.
The electrode base metal and the noble metal chip have sufficient heat resistance. However, in some cases, as a result of exposure to a high-temperature oxidizing condition and high-temperature heat cycles, cracking has occurred at a joint portion between the electrode base metal and the noble metal chip, and the cracking has progressed, leading to separation or detachment of the noble metal chip. Also, with recent practice of lean burn of fuel and higher degree of compression, a reduction in diameter of the noble metal chip has been required, and the electrode temperature is showing a tendency to increase. As a result, load imposed on the joint portion between the electrode base metal and the noble metal chip is increasing; thus, the noble metal chip is more apt to be separated or detached from the electrode base metal. In order to cope with the situation, various attempts have been made to strongly join the electrode base metal and the noble metal chip.
Japanese Patent No. 3121309 specifies the dimensions of a weld metal layer formed between a noble metal chip and a center electrode or a ground electrode for providing a high-performance, long-life spark plug having excellent strength of joining at the weld metal layer for an internal combustion engine.
Japanese Patent No. 3702838 specifies the shape of a weld metal zone where a noble metal chip and a ground electrode are fused, and the dimensions and components of the noble metal chip in order to provide a spark plug which exhibits enhanced joint performance between the ground electrode and the noble metal chip while ensuring ignitability.
Japanese Patent Application Laid-Open (kokai) No. 2002-237370 specifies the dimensions of a full-circle laser weld zone which extends into a noble metal chip and into a chip mounting surface region, for providing a spark plug having enhanced durability of an ignition portion.
Meanwhile, a ground electrode is disposed deeper in a combustion chamber than is a center electrode. Thus, the ground electrode becomes higher in temperature than the center electrode; i.e., the ground electrode is disposed in a severe environment involving great temperature fluctuations. Therefore, there has been greater demand for measures to prevent separation or detachment of the noble metal chip from the ground electrode.
An object of the present invention is to provide a spark plug in which an ignition portion of a ground electrode formed through joining of a noble metal chip to the ground electrode has high durability, and a method of manufacturing the spark plug, and particularly, to provide a spark plug which exhibits good joint performance between the noble metal chip and an electrode base metal of the ground electrode, and a method of manufacturing the spark plug.